Airport Base Station Poor Reception

What could cause my base station to suddenly provide a poor signal? I used to have great coverage in my house, upstairs, on my deck, etc. My base station is located on my first floor, and have just recently (past 3 weeks) noticed very poor reception. Sometimes I'll turn off Wifi on my iPhone so I'll get LTE, but I need to be on my network for Apple TV and Air Play, etc. Nothing else has changed in my house configuration. No new devices. No new Airport Express stations (I've had one for years). Any ideas? Could the base station just have something wrong with it? I do all the updates when asked.

Not having luck changing the channels or GHz. Nothing is helping. But have a question. If I have a couple Airport Express off my Airport base station, wouldn't I want them all set up to Extend a Network rather than Join a Network? Is there any harm in that? Will that do what it sounds like it will do?

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